1. Field of the Invention
The present invention relates to a machine tool having at least one tool spindle which is travelling in a work space and on which a tool receptacle for clamping tools is provided, and having a magazine for tools which can be introduced into the tool receptacle in the tool spindle, the magazine having a transport chain which can be moved back and forth in a transport direction and on which carriers are arranged for receiving tools.
2. Related Prior Art
Such machine tools are frequently known from the prior art.
The known machine tools serve to produce workpieces, on which various operations have to be carried out with different tools in one set-up. For this purpose, the workpiece is mounted in place in a fixture on a workpiece table which is arranged in the work space of the machine tool.
The machine tool has a tool spindle which is travelling relative to the workpiece table in three coordinate directions and on the end face of which a tool receptacle is provided, in which the tools can be clamped in place for machining the mounted workpiece. In order to permit rapid machining of the workpiece, the tools are changed automatically, for which purpose a magazine is provided on the machine tool, in which magazine the various tools required during the machining of the workpiece are kept available.
In particular when the number of tools required is very high, so-called chain magazines are used, in which the individual chain links are designed as carriers for the tools. The carriers thus form a transport chain, which can be designed as an endless chain and is moved back and forth in a transport direction. In this way, it is possible to bring the tool required next in each case into its transfer position relative to the spindle by moving the transport chain either in or against the transport direction. This movement of the transport chain is also referred to as indexing.
During a tool change, first of all an empty storage space is moved up to the transfer position, whereupon the tool used last is deposited in this empty storage space. The transfer between the tool receptacle in the tool spindle and the storage space in the transport chain is effected, for example, by means of a tool changer, which has a gripping arm for gripping the tool at a collar specially provided for this purpose, whereupon the tool is then ejected from the tool spindle. The gripper deposits the tool in the storage space, where it is held by spring pressure.
On the other hand, it is also possible to dispense with the tool change and to approach the storage spaces directly with the spindle. To this end, the front end of the magazine is moved into the work space, or the spindle is moved out of the normal working region and above of the chain magazine, so that the spindle can be positioned above the storage space. This type of tool change is also referred to as “pick-up”.
After the tool which has been in use beforehand has been deposited in an empty storage space in this way, the magazine is indexed, so that the storage space having the tool which is to be introduced next is moved into the transfer position. This transfer position is provided at the front end at the chain magazine, which is moved in an arc into the work space and out of the work space again.
When the new tool has been inserted into the spindle either by the pick-up method or by means of a tool changer, the chain magazine is retracted out of the work space or is provided with an appropriate protective cover. In this way, chips which are produced during the machining of the workpiece are prevented from contaminating the magazine and/or the tools mounted in the magazine.
In the known machine tools, it is therefore always necessary to index the magazine for the tool change. In the most unfavourable case, the tool to be used next is located at the opposite end of the transport chain, so that the storage space with the tool which is next to be introduced as replacement has to move along half the possible traverse path until it reaches the transfer position. Provided the storage space with the new tool is further away from the transfer position in the transport direction than half the traverse path, the transport chain is moved against the transport direction, as a result of which a distance shorter than half the transport path has to be covered.
In the machine tools and magazines described in this respect, it is disadvantageous that the average tool change time depends on the length of the transport chain and thus on the maximum number of tools to be stored. The more storage spaces such a transport chain has, the longer it is and the more time is required for a tool change from the most unfavorable storage position.
The known arrangement is in particular disadvantageous when tools having extremely different dimensions are to be used. This is because the tool having the maximum dimensions then determines the size of a chain link which has to be designed as a carrier and thus as a storage space for the tools. Since all the chain links have the same dimensions for reasons of drive technology, guidance and other kinematics, the storage density for tools is extremely small in the known chain magazines. In other words, there is a lot of empty space between individual storage spaces in order to take into account the possible dimensions of extremely large tools.
On account of the high travelling speed of the tool spindle relative to the workpiece, the machining operations to be carried out with a tool can be performed in an extremely short time. Compared with these short machining times, the tool change time is relatively long for the abovementioned reasons, which is regarded on the whole as a disadvantage.
This long tool change time results in particular from the small storage density of the known chain magazines.
However, such chain magazines are used not only in single-spindle machines but also in so-called two-spindle machines in which two workpieces are machined simultaneously with two tool spindles which are arranged on a common carrier. The two tool spindles are at a defined distance from one another, the workpieces being mounted on the workpiece table at precisely this defined distance apart. In this way, it is possible to machine two workpieces simultaneously if the same tool is clamped in place in each tool spindle.
If it is desired to keep a large number of available tools in such two-spindle machines, the chain magazines discussed above are also used in two-spindle machines. In this case, it is possible, on the other hand, to assign a separate tool magazine to each tool spindle, which is complicate in terms of design and is accordingly costly. Furthermore, the disadvantages discussed above also then arise for two-spindle machines.
On the other hand, it is also possible to jointly assign one chain magazine to both tool spindles. If the tool change is to be effected by the pick-up method in this case, the individual storage spaces must be at a distance apart which corresponds to the center distance of the two tool spindles. This means that the turn around arc of the magazine in the work space must be designed to be of corresponding width, as a result of which the contour of the magazine gets in the way in the work space or can only be moved into the work space and out of the latter again in a complicated manner. Furthermore, the distance between the two tool spindles, which is around 250 mm for example, determines the storage density of the chain magazine. Due to the large distances between the individual storage spaces on the one hand, and because storage spaces now have to be provided in each case for a tool change on the other hand, the overall length of the magazine is considerably increased once again, which has the described adverse effects on the tool change time.